When the thickness of a coated plate is measured, a suitable thickness measuring instrument is generally employed to measure the thickness after the coating is stripped off. However, since the work of removing the coating and repainting takes a great deal of labor, means for measuring the thickness of the plate with the coating intact have been heretofore studied.
FIG. 1 shows an example of such a conventional thickness measuring means, which measures the thickness of a plate by utilizing the reflection of an ultrasonic pulse. This thickness measuring means has an ultrasonic probe 10 used in close contact with a coated plate 12, and a thickness measuring unit 14 for measuring the thickness of the plate 12 according to a signal from the ultrasonic probe 10. The ultrasonic probe 10 is a so-called "dual type", and has an ultrasonic pulse transmitting element 16 and an ultrasonic pulse receiving element 18 aligned next to each other in a cylindrical case 20. An acoustic partition plate 22 extends between the transmitting element 16 and the receiving element 18 to the opening face of the case 20 so that an ultrasonic pulse from the transmitting element 16 does not propagate directly to the receiving element 18. A delaying material 24 is filled in the case 20. This ultrasonic probe 10 is used in close contact at the end face thereof with the surface of the coated plate 12. When an ultrasonic pulse is generated from the transmitting element 16, the pulse is reflected at the boundary face between a coating 26 and a plate 28 and the bottom of the plate 28, reflected waves are received by the receiving element 18, and its received signal is sent to the thickness measuring unit 14. The thickness measuring unit 14 calculates the thickness of the plate in accordance with the difference in reception times between the reflected wave from the boundary face and the reflected wave from the bottom of the plate.
However, since the acoustic partition plate 22 of the ultrasonic probe 10 of this means contacts with the surface of the coated plate 12, there arises a problem that the reception level of the reflected wave from the boundary face is low and measurement errors can easily occur. More specifically, in order to produce a signal by the reflected wave from the boundary face, it is necessary to greatly amplify the signal, and signal errors arising from the amplification cannot always be ignored. Simultaneously, noise which causes measurement errors (including various waves reflected within the coating 26 in the reflected wave from the boundary face, waves reflected from the top surface, etc.) is easily picked up. Therefore, particularly when the coating is thin, it is difficult to select and positively produce a signal from the reflected wave from the boundary face, and it is also difficult to accurately obtain the true thickness of the plate 28.
As another means, there is known a so-called B.sub.1 -B.sub.2 method utilizing multiple bottom reflection waves. As shown in FIG. 2, this B.sub.1 -B.sub.2 method obtains the thickness of a plate by measuring the time difference between the reflected wave (a first bottom reflection wave signal B.sub.1) reflected from the bottom of a plate 28 and a bottom reflection wave (a second bottom reflection wave signal B.sub.2) in which part of the first bottom reflection wave is not transmitted through the boundary face between the coating 26 and the plate 28 but is reflected and then reflected again from the bottom of the plate 28.
However, as shown in FIG. 3 in the B.sub.1 -B.sub.2 method, if a corroded portion 30 exists on the bottom of the coated plate 12, signal f.sub.1 from the reflected wave of the corroded portion 30, a signal f.sub.2 of another reflected wave by the reflected wave and a signal B.sub.1 of the bottom reflection wave are mixed in the signal output from the receiving element 18 of the probe 10, these signals cannot be clearly identified, and it is sometimes impossible to accurately measure the thickness of the plate 28.